Semiconductor image sensors are used to sense radiation such as light. Complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) and charge-coupled device (CCD) sensors are widely used in various applications such as digital still camera or mobile phone camera applications. These devices utilize an array of pixels in a substrate, including photodiodes and transistors that can absorb radiation projected toward the substrate and convert the sensed radiation into electrical signals. A back side illuminated (BSI) image sensor device is one type of image sensor devices. These BSI image sensor devices are operable to detect light from its backside.
The conventional sensor, called the “front side illumination (FSI)” image sensor for these CMOS chips, is constructed in a fashion similar to the human eye, and has a lens at the front, layers of metal having wiring in the middle, and photo detectors on a silicon substrate (which absorbs the light) at the back. These metal layers may not only deflect the light on the sensor, they could also reflect it, reducing the incoming light captured by the photo detectors. By contrast, the back side illuminated sensor has the same elements as FSI, but orients the wiring behind the photo detectors layer by flipping the silicon wafer during manufacturing and then thinning its reverse side so that light will hit the silicon first, and the photo detectors layer without passing through the wiring layer. This change can improve the chance of an input photon being captured from about 60% to over 90%, and the sensitivity per unit area to deliver better low-light shots.
A BSI image sensor device typically has a device region of a wafer and a bond pad region. One of the failure mechanisms known for the bond pad is cratering, which is a mechanical damage to the bonding pad. Cratering happens when the bond pad is peeled off, by some upwardly pulling force, along with a chunk of silicon and in many cases part of active circuit underneath the bond pad, thereby creating a ‘hole’ or ‘crater’ on the silicon substrate. It causes a partial or total fracture of the silicon material underneath the bond pad. Bonding time, force and power are critical parameters when dealing with cratering.
Such cratering or bonding pad peeling not only degrades, but seriously damages BSI image sensor device performance. Therefore, it is desirable to provide a method of designing and manufacturing a bond pad structure in an image sensor device, such as BSI, such that any excess stresses that might be potentially applied to the silicon substrate layer may be adequately released or redistributed to prevent the cratering or bond pad peel-off problem.